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Date:   Mon, 13 Feb 2023 01:55:04 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Jan Kara <jack@suse.cz>
Cc:     Matthew Wilcox <willy@infradead.org>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-mm@kvack.org, John Hubbard <jhubbard@nvidia.com>,
        David Howells <dhowells@redhat.com>,
        David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH 1/5] mm: Do not reclaim private data from pinned page
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On Fri, Feb 10, 2023 at 12:29:54PM +0100, Jan Kara wrote:
> functionally that would make sense but as I've mentioned in my reply to you
> [1], the problem here is the performance. I've now dug out the discussion
> from 2018 where John actually tried to take pinned pages out of the LRU [2]
> and the result was 20% IOPS degradation on his NVME drive because of the
> cost of taking the LRU lock. I'm not even speaking how costly that would
> get on any heavily parallel direct IO workload on some high-iops device...

I think we need to distinguish between short- and long terms pins.
For short term pins like direct I/O it doesn't make sense to take them
off the lru, or to do any other special action.  Writeback will simplify
have to wait for the short term pin.

Long-term pins absolutely would make sense to be taken off the LRU list.